Vintage threads from the first ten years

Re: Another spitfire question: OT

The measured primary impedance on a Spitfire OT was around 4K (I think Doc measured 3.8K). The blues deluxe OT that you are using probably has a primary Z of 8K or so. By connecting a 4 ohm speaker to the 8 ohm tap, you are effectively lowering the primary Z from 8K to 4K. The same result could be obtained by connecting an 8 ohm speaker to the 4 ohm tap.

Why does it sound better? For a given tube type, there is an optimum impedance that provides a reasonably high level of output power with a reasonably low level of harmonic distortion. The 8K value is what the tube manufacturers specified as the compromise to achieve a good power to distortion ratio for the El84/6BQ5 (for hi-fi, not guitar!). By lowering the primary Z, distortion and power are both increased. It appears that Mark Sampson, the designer of Matchless amps, did a lot of experimenting with this to find the optimum value for guitar. I have played around with this quite a bit myself, and I agree with your findings - lower primary Z results in a 'beefier' tone, for lack of a better word. Switching back to 8K thins the tone out. There is probably a better technical explanation for exactly what's going on here, so hopefully someone can shed some more light on this. From a purely practical point of view I design all my EL84-based amps with a 4K primary Z, since the tonal difference is significant, at least to my ears. Hope this helps...